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Chemistry: The Effects of Intermolecular Forces

The Effects of Intermolecular Forces

The intermolecular forces present in a compound play a role in that compound's properties. This isn't really surprising when you think about it. After all, if the molecules in one liquid are held tightly together by a strong intermolecular force, this liquid would be expected to behave differently than a second liquid in which the molecules are held together very weakly. The following are two of the ways in which intermolecular forces affect the properties of a liquid:

  • Melting and boiling point: Generally, compounds that undergo hydrogen bonding melt and boil at higher temperatures than compounds that experience dipole-dipole forces or London dispersion forces. For example, let's consider the following three molecules.
  • Surface tension: Liquids with stronger intermolecular forces tend to have higher surface tension than those with weak intermolecular forces. For example, if you pour a very small amount of water on a table, it will tend to collect together in one large drop. On the other hand, if you do the same thing with lighter fluid, which has weaker intermolecular forces, you'd find that the lighter fluid tends to spread out over a larger area. For those of you who were planning on pouring lighter fluid on your coffee tables, please only do so under the supervision of a grown-up!
Molecular Meanings

Surface tension is the tendency of liquids to keep a low surface area. For example, if you fill a cup of water to the top, you'll find that the water can actually rise above the lip of the cup slightly. The reason it doesn't spill is that the water molecules at the surface hold together like the surface of a balloon, holding the water inside. Just don't try to move the cup!

Compound Intermolecular Force Melting Point Boiling Point
CH4 London dispersion force -182º C -164º C
HCl Dipole-dipole -115º C -85º C
H2O Hydrogen bonding 0º C 100º C
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Excerpted from The Complete Idiot's Guide to Chemistry © 2003 by Ian Guch. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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